Recently, the need for a data communication network at high speed and of a large capacity is constantly increasing. Accompanied therewith, as an indoor or in-car data communication network, there has been increasing interest in a data communication network using a plastic optical fiber (POF). A light source of this data communication network is required to have high power and high-speed response characteristics. As a light emitting element having these two characteristics, there is a surface light emitting element (for example, a light emitting diode) having a resonator structure.
The surface light emitting element having the resonator structure has the structure (vertical resonator) which resonates light from an active layer in a vertical direction with respect to the active layer by sandwiching the active layer between two reflective layers. In the vertical cavity structure, when the active layer is to be a quantum well layer, high-speed response characteristics are achieved. Furthermore, the formation of a plurality of quantum well layers allows a higher light emission output.
In Patent Document 1, disclosed is a light emitting diode in which an active layer is to be a single quantum well layer with small thickness, and impurities are doped into two barrier layers sandwiching the quantum well layer therebetween and the quantum well layer, thereby improving response characteristics.
In Patent Document 2, disclosed is a light emitting diode in which the length of the resonator is set to be half of a light emission wavelength, a plurality of quantum well layers is provided at the central portion of the resonator, and thereby a plurality of the quantum well layers lies in positions to be an anti-node of a standing wave in the resonator. In this light emitting diode, there is included on both sides of each of the plurality of the quantum well layers tunnel barrier layers. Further, the thickness of each pair of the tunnel barrier layers is different from the thicknesses of any other pair of the tunnel barrier layers. Owing to this structure, wider carrier levels due to bias of the number of carriers between respective quantum well layers are suppressed, and QED effects of the resonator are enhanced and light emission efficiency is increased. Incidentally, a bandgap-matching layer is formed between the tunnel barrier layers.
In Patent Document 3, disclosed is a light emitting diode of a high light emission efficiency in which a plurality of quantum well layers having peak wavelengths of an emission spectrum different from each other is positioned at intervals of ½ a resonance wavelength in positions of an anti-node of a standing wave in a resonator, and whereby the carrier that gets over one quantum well layer contributes to the light emission at the next quantum well layer.
In the vertical cavity light emitting diode disclosed in Patent Document 4, the length of a resonator is ½ the light emission wavelength, a quantum well layer is provided at the center of the resonator thereof, and whereby the quantum well layer lies in a position at an anti-node of a standing wave of light; as well as the bandgaps of these respective quantum well layers are formed to be inclined or stepped, being smaller from the injection side of electrons toward the injection side of electron holes, and whereby the electrons and the electron holes are localized in the same place in respective quantum well layers to enhance spontaneous emission.
In Patent Document 5, disclosed is a vertical cavity light emitting diode which provides the quantum well layers having bandgaps different from each other in an inclined manner to form a multiple quantum well structure, whereby widening a gain bandwidth.
In Patent Document 6, disclosed is a vertical cavity light emitting diode which has quantum well layers with the same bandgap, disposes a plurality of quantum well layers having thicknesses different from each other, sets the interval of the quantum well layers to be ½ the resonance wavelength, thus forming no new levels due to coupling between respective quantum well layers, and keeps the emission spectrum of respective quantum well layers.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2002-111053
[Patent Document 2] Japanese Patent Application Laid-Open No. 2000-174328
[Patent Document 3] Japanese Patent Application Laid-Open No. 10-27945
[Patent Document 4] Japanese Patent Application Laid-Open No. 2000-174327
[Patent Document 5] Japanese Patent Application Laid-Open No. 7-245449
[Patent Document 6] Japanese Patent Application Laid-Open No. 10-27945